This project is designed to obtain a better understanding of the immunological reactions which occur in animals infected with Cryptococcus neoformans and in animals who exhibit protective immunity to the organism. Animals immunized with a pseudohyphal variant of C. neoformans will be compared to animals who will succumb to cryptococcal infection. Lymphocyte and macrophage functions will be examined, as well as any lymphocyte-macrophage interactions which might occur. Lymphocyte subsets will be examined to determine which activities are necessary for protection against cryptococcal infection and which lymphocyte subsets exhibit the functions. Particular attention will be given to the possible occurrence of suppressor cells in infected animals since this cell subset may contribute to the very weak immune responses which occur in patients with cryptococcosis.